Circuit for effective quench heating in superconducting magnets

ABSTRACT

An assembly comprising a number of superconductive coils, at least one quench heater arranged to heat the superconductive coil(s) in the event that at least part of at least one of the coils enters a quenched state; means for transferring energy from the coil(s) to the heater(s) in the event that at least part of at least one of the coils enters a quenched state; wherein the means for transferring energy from the coil(s) to the heater(s) includes a series capacitance, through which the energy transferred must pass.

The present invention relates to superconducting coils. Moreparticularly it relates to apparatus for preventing damage tosuperconducting coils in the case of a quench.

Superconducting coils are used in a variety of applications, for exampleas magnetic field generators in MRI (Magnetic Resonance Imaging) or NMR(nuclear magnetic resonance) equipment. Coils of superconducting wireare held at cryogenic temperatures, typically at about 4K, thetemperature of boiling helium. So-called high-temperaturesuperconductors still require cryogenic temperature of the order of100K. An ever-present risk in the use of superconductive coils is therisk of a quench. For a reason such as a localised heating, thetemperature of a region of the superconducting wire rises above itscritical temperature. That region becomes resistive. The current flowingthrough the coil continues to flow through the resistive region, andheat is accordingly dissipated. This heat causes a larger region of thesuperconductor to become resistive, increasing the heat dissipated.Since the resistive region is initially small, the heat dissipation isconcentrated in a small volume. The temperature of this small volume mayaccordingly rise to such a temperature that the superconductive coil isdamaged. When a known MRI superconducting magnet quenches, an energy ofthe order of 8 MJ must be dissipated in a time period of 2-3 seconds.

It is known to avoid such damage by providing quench heaters. Inresponse to a region becoming resistive, energy is diverted toelectrical heaters placed adjacent to other regions of thesuperconductive coils. With one coil, or one part of a coil, in aquenched state then a resistive or inductive voltage is built up acrosseach coil. This inductive or resistive voltage is applied to the heatersto induce a quench in the other coils. These heaters heat thecorresponding parts of the superconductive coils above their criticaltemperature, and those regions also become resistive. The effect of thisis that substantial regions of the coils become resistive, so that heatis dissipated over a much larger region of the coils, meaning that hightemperatures are not reached, and the coils are not damaged. Typically,this is achieved by supplying the voltage developed across the firstcoil to quench to small foil heaters, such as NiCr alloy foil coilsplaced on each superconducting coil. These heaters may each supplyapproximately 2 W of heat power to the associated superconductive coil.

It is generally desired to minimise the amount of superconducting wireemployed in the coils. This minimises the cost of the resultant system,and reduced the chances of a quench-inducing defect being present in thecoil. The cross-sectional area of the wire may also be reduced in anattempt to save cost, weight and size of the resultant system. Suchreduced area wires will be more susceptible to damage from overheating.However, if the heaters are capable of reacting very quickly to aquench, the coils may be placed in a resistive state quickly enough toavoid damage, even to superconducting wires of reduced cross-section.

FIG. 1 shows a circuit diagram of a known arrangement of quench heaters.A series connected set of superconducting coils 10 are individuallylabelled A to H. A power source 12 may be connected to the coils tosupply a current. One end of the series connection of coils is grounded.Back-to-back diodes 14 and a superconducting cryogenic switch 18 areplaced between the ends of the series connection of coils to provide acurrent path for current through the superconducting coils when thepower source 12 is removed. An array of heaters 16 is provided. Althoughillustrated as separated away from the coils 10 for clarity, the heaters16 would be arranged in close physical proximity with the coils 10.

All superconducting magnets which are operated in the so-calledpersistent mode have a cryogenic switch. Essentially, it is a piece ofsuperconductor wire, in series with the magnet coils 10, with a heaterattached to it. If the heater is on, the cryogenic switch 18 is normallyconducting and is open. When the system is attached to an external powersupply by leads 12, current will flow through the superconducting coils10, with only a trickle running through the cryogenic switch 18. Oncethe magnet system has been ‘ramped’ to the required current, the heateris turned off, and the cryogenic switch 18 becomes superconducting: thecryogenic switch is closed. As the external power supply connected toleads 12 is ramped down, the current through the cryogenic switch 18will increase by the same amount as the decrease in the current throughthe external power supply. Once the external power supply is ramped downcompletely, the current leads 12 may be removed, to limit heat leakageinto the cryogenic magnet system.

The heaters 16 are shown in FIG. 1 as connected in two parallelbranches, each branch including a series connection of heaters. Otherconnection arrangements of the heaters may of course be used, dependingon factors such as the voltages developed across the coils during aquench, the current handling requirement of the heaters, and theresistance and power handling capacities of the heaters. From aconsideration of this circuit arrangement, it is clear that the polarityof voltage V which is developed across the heaters in the case of a coilquench will depend on which coil quenches first. In the illustratedexample, if one of coils A-D quenches first, then the voltage V will benegative with respect to the ground shown. On the other hand, if one ofcoils E-H quenches first, then the voltage V will be positive withrespect to the ground shown. The heater arrangement must be capable ofcoping with either polarity of applied voltage V. The heaters are linkedto the coils through a back-to-back arrangement of diodes 20 at one end.The superconducting coils 10 are shown in FIG. 1 as connected in series,with the heater arrangement connected between a ground terminal 22 and apoint halfway along the series connection of coils. The ground terminalis also linked to one end of the series connection of the coils.

The back-to-back diode arrangement 20 connecting the heaters 16 to thecoils 10 provides a threshold voltage which must be exceeded before anycurrent flows through the heaters 16. This threshold voltage should bechosen such that current does not flow through the heaters during rampup, that is, while current is being established in the superconductingcoils, but such that the voltage drop across the diodes 20 is not sohigh that the effectiveness of the heaters will suffer during a quench.In current systems, the threshold voltage is in the order of 5-10V.

In known systems, the heaters 16 may each have a resistance of 120 Ohms.Typically, the heater may be required to provide a power of 2 W under amean applied voltage of 15V. The heater arrangement 16 must be capableof withstanding the full quench voltage applied across it. This quenchvoltage may reach 5 kV, and the heaters must be capable of toleratingsuch voltage without burning out. In known systems, this limitation hasbeen approached by adding more heaters in series. This introducesfurther problems in that the heaters each take longer to reach their 2 Wpower output. There will be an increased delay before the coils will bequenched by the heaters. During this delay, there is a risk of damage tothe superconductive coil, since the quench will remain localised, and anexcessive heat build-up may occur in that region.

The curve in FIG. 2 shows the development of V, the voltage across theheater circuit 16, with time following the start of a quench. Thevoltage V initially rises to a high value as the large currentscirculating in the superconducting coil pass through the resistive,quenched, part of the coil. This causes heating of the superconductivecoil, leading to greater resistance and greater dissipation of energy.The increasing voltage V eventually levels out to a maximum voltageVmax, which may be of the order of 5 kV.

The voltage V reaches a peak value V_(max) a certain time t_(max) afterthe start of a quench. The time t_(max) may be of the order of twoseconds. The values V_(h) of voltage are the minimum voltage required toheat the heaters sufficiently to cause quench in the heated coils. Thevoltage V exceeds V_(h) between times t_(h1) and t_(h2). This time mustbe sufficiently long to ensure effective quench of the heated coils. Thedissipation of energy within the coils continues, and the voltage Vacross the heaters falls. The heat dissipated by a heater isproportional to the square of the voltage across it.

One solution which has been proposed is to reduce the resistance of theheaters 16 so that the required heat dissipation may be reached morequickly. One disadvantage of this is that the heaters themselves may bedamaged by the quenching voltage. In the arrangement illustrated in FIG.1, a quench voltage of 5 kV may attempt to dissipate dissipate 3 kW in a120Ω heater designed to dissipate a maximum of 400 W. This could damagethe heaters.

The present invention accordingly provides apparatus as defined in theappended claims.

The above, and further, objects characteristics and advantages of thepresent application will become more apparent from consideration of thefollowing description of certain embodiments, given by way of examplesonly, with reference to the accompanying drawings wherein:

FIG. 1 shows a schematic diagram of superconducting coils equipped withquench heaters, according to the prior art;

FIG. 2 shows the development of voltage across the heater circuit in thesystem of FIG. 1;

FIG. 3 shows a schematic diagram of an arrangement similar to that ofFIG. 1, modified according to the present invention; and

FIG. 4 shows the development of voltage across the heater circuit ofFIG. 3, and the square of the time differential of that voltage.

According to the present invention, the heaters 16 are capacitivelycoupled to the superconducting coils, rather than being DC coupled. FIG.3 shows an example of a circuit according to an embodiment of thepresent invention, in which a capacitor 30 is placed in a diode bridgerectifier 34 linking the two intermediate nodes in place of the DCconnection normally provided. This particular placement of the capacitorenables a polarised capacitor to be used, since the voltage on thecapacitor will always be of a certain polarity. In other embodiments ofthe invention, a non-polarised capacitor is employed, and it may beplaced at any position in series between the heaters and the coil, orground, connections. In such embodiments, the bridge rectifier 34 is notrequired.

FIG. 4 illustrates an advantage of capacitively coupling the heaters,according to the present invention. The upper curve in FIG. 4 shows thedevelopment of V, the voltage across the heater circuit 32, with timefollowing the start of a quench. The voltage V initially rises to a highvalue as the large currents circulating in the superconducting coil passthrough the resistive, quenched, part of the coil. This causes heatingof the superconductive coil, leading to greater resistance and greaterdissipation of energy. The increasing voltage V eventually levels out toa maximum voltage Vmax, which may be of the order of 5 kV. Thedissipation of energy within the coils continues, and the voltage Vacross the heaters falls. The heat dissipated by a heater isproportional to the square of the voltage across it. The lower curveshows the development of the square of the rate of change of the voltageV with time t, |dV/dt|², with time t. Since, according to the presentinvention, the heaters are capacitively coupled to the coils, the powerdissipated by each heater will be proportional to |dV/dt|². As shown inFIG. 4, |dV/dt|² reaches a peak value at time t′_(max), which is rathersooner than the time tmax of the peak value of voltage V. The peakheating output from the heaters is accordingly reached at time t′_(max).The fact that this occurs earlier than t′_(max) in the case of DCcoupled heaters (FIG. 2) means that the heaters can quench theassociated coils correspondingly earlier, and so reduce the likelihoodof damage to the superconducting coils. To induce a quench in asuperconductive coil, it is typically sufficient to heat the coil at atleast 2 W for 0.3 seconds. As illustrated by the curves in FIG. 4,heaters arranged according to the present invention reach this level ofheating more rapidly than those of the prior art, leading to a morerapid quench.

In FIG. 4, h represents the minimum level of |dV/dt|² required toproduce the heating required to quench the heated coils. Sufficientpower is provided between time t′_(h1) and t′_(h2). The time periodbetween points t′_(h1) and t′_(h2) must be sufficient to ensure aneffective quench. According to a benefit of the present invention, thetime t′_(h1) occurs significantly earlier than the time t′_(h1) in FIG.2. The heated coils will be quenched sooner than in the known system,leading to reduced likelihood of damage to the superconductive coils.

The present invention allows all coils to be quenched sooner than withknown arrangements which means that the required energy dissipation maybe more effectively spread across all of the coils. This in turn meansthat each coil need only be designed to tolerate a reduced maximumenergy dissipation. This allows thinner copper cladding to be providedaround the superconducting conductor in each coil, in turn allowing fora cheaper, lighter and smaller magnet as a result.

The capacitor 30 used must be of a relatively high value, and be capableof withstanding relatively high voltages. For example, a capacitor of 47μF capacitance with a voltage rating of 5 kV may be suitable. Certaintypes of film capacitor may be suitable to fulfill this role, and maytolerate operation at cryogenic temperatures. Electrolytic capacitorsare available in appropriate capacitance and voltage ratings. However,such capacitors may be unsuited to being located inside thesuperconducting coil system, at cryogenic temperatures. An electrolyticcapacitor may be provided for the purposes of the present invention onthe outside of the superconducting coil system, but would requirecareful provision of a high voltage connecting cable, and steps wouldneed to be taken to avoid disconnection of the high voltage connectingcable.

The value of the capacitor 30 can be selected to suit the heaterresistances and the energy required to be dissipated. When the capacitorhas fully charged, that is, once the quench voltage is at its peak, thedissipation of the heaters returns to zero. For a capacitor of 50 μFcharged to 4 kV, the energy stored in the capacitor is q=½CV²:50×10⁻⁶×2000²=100J.

The value of the capacitor should be selected to provide the requiredperformance. A larger capacitance will allow greater energy storage,required for providing enough heat to quench the coils, while a smallercapacitance will provide the same peak power dissipation, but for ashorter time. This shorter time may be advantageous in preventing damageto the heaters.

1. An assembly comprising: a number of superconductive coils, at least one quench heater arranged to heat the superconductive coil(s) in the event that at least part of at least one of the coils enters a quenched state; means for transferring energy from the coil(s) to the heater(s) in the event that at least part of at least one of the coils enters a quenched state; wherein the means for transferring energy from the coil(s) to the heater(s) includes a series capacitance, through which the energy transferred must pass.
 2. An assembly according to claim 1 wherein a plurality of superconductive coils are connected in series, and the heater(s) is/are connected across a subset of the coils.
 3. An assembly according to claim 2, wherein the heater(s) is/are connected across one half of the coils.
 4. An assembly according to claim 1, wherein the heater(s) is/are connected to the superconductive coils through a bridge rectifier, and wherein the series capacitance is provided between intermediate nodes of the bridge rectifier in place of a DC connection. 